Translation type isolation bearings, or bearings that are relatively rigid in one direction and relatively flexible in the two orthogonal directions, consist of at least a core of horizontally sliding or translating surfaces or materials that reduce the transmitted vibration energy. At the same time, the core is also capable of carrying the vertical gravity loads of the structure.
In the case of seismic isolation bearings, movement at the base of the bearing is associated with earthquake ground motion. The maximum possible translation that might be imposed on the isolation devices is not well understood because there is disagreement on the maximum potential ground motions. It is believed that the worst case ground motion would be adjacent to the terminating end of a slipping fault line. But incomplete understanding of the potential effects of geologic factors, such as fault size and type, as well as limited localized information, such as fault locations or soft soil conditions, reduce the accuracy of maximum bearing displacement estimates at a specific site. Therefore, it is considered prudent to guard against unexpected bearing failure, caused by unexpectedly high bearing displacements, that may lead to sudden loss of structure support.
At present, control of maximum bearing displacements is accomplished by utilizing a variety of means, including: (1) designing overly stiff bearings; (2) relying on increasing elastomer stiffness with strain; (3) adding hydraulic type dampers or pistons in parallel with the bearings; (4) providing metal chains, cables or rods to stop the bearings; (5) providing auxiliary friction sliding devices that provide increasing friction resistance with bearing translation; and, (6) providing stabilizing columns that "catch" the building if the bearings should fail.
Representative bearings having maximum bearing displacement control include U.S. Pat. Nos. 4,910,930 (Way) and 5,014,474 (Fyfe et al.). Way discloses a seismic isolation structure comprised of a high damping elastomeric bearing and a restraint means mounted between a building's footings and support columns. This restraint means is comprised of a curled steel rod located outside the bearing core. Fyfe discloses an apparatus having a low friction elastomeric load bearing pad disposed between an upper and lower load bearing plate and a freely disposed restraining means, such as a steel cable or chain, in an axial bore through the center of an elastomeric bearing and attached to the upper and lower load bearing plates.
Although capable of handling unexpected inputs, conventional displacement controlled bearings do not exhibit an optimized design capable of handling expected inputs. In the case of a displacement controlled seismic isolation bearing, an optimum bearing design would entail one in which structure accelerations are reduced as much as possible for typical, expected ground motions ranging in scale from Richter Magnitude 5 to 7, and failure is prevented in extreme, unexpected ground motions, or those greater than Richter Magnitude 8.
Bearings having displacement control, to date, are deficient in their design and function for a number of reasons, including: (1) overly stiff bearing cores reduce isolation effectiveness in the expected input range; (2) elastomer stiffness increase with strain is too slow to prevent displacement bearing failure; (3) hydraulic or similar viscous systems are expensive and overly stiffen the bearing, reducing its effectiveness; (4) friction sliding devices tend to be unreliable because they depend on long-term, consistent stick-slip action at the sliding surfaces and are sensitive to normal or vertical force which varies in a dynamic and complicated way; and (5) steel cables, chains, springs or rods generate a sudden impact load on the structure when they become taut and typically do not return to their initial configuration after becoming taut.